The invention concerns the sealing of telescopically related components to provide for relative reciprocating motion therebetween while effecting a seal to minimise fluid leakage between the relatively moving surfaces of the elements. The invention is applicable in a number of fields but is especially applicable in the field of fluid- (e.g. pneumatic-) powered tools such as hammer drills, especially important applications being in the field of so-called down-the-hole hammer drills used for rock drilling and the invention will be described in relation to applications in that field.
A typical down-the-hole hammer drill used for rock drilling purposes consists of a drill body within which a piston is caused to reciprocate to deliver blows upon an anvil surface at the end of the shank of a bit that protrudes from the drill body and is provided with a cutting face having such a configuration as to form a bore hole suitably larger than the drill body so as to enable this to follow the bit into the bore hole as the latter increases in length. In order that the hammer blows delivered by the piston to the bit shank may be effectively transmitted to the cutting face of the bit, and thus to the rock or other formation with which the bit is engaged, the bit must have some freedom of axial (longitudinal) movement with respect to the drill body: thus the drill body must include parts that coact with the bit shank to hold this suitably aligned with the body while permitting the required degree of relative motion. These bit-retaining and locating arrangements must usually provide for the transmission of torque between the drill body and the bit shank in order that rotation of the drill body may be imparted to the bit.
Because the anvil surface of the bit shank is exposed to blows by the piston, that end of the bit shank communicates with the working chamber to which pressure fluid--usually compressed air--is admitted to cause movement of the piston away from the bit shank after delivering a blow upon the anvil surface. Although provision is often made for exhausting the fluid from this chamber through passages in or adjacent to the bit shank, uncontrolled leakage of fluid from the said chamber represents a loss of energy and results in inefficiency in the conversion of pressure fluid energy into drilling effect. Accordingly, provision must be made to minimise such wasteful leakage of pressure fluid along the bit shank.